1. Field of the Invention
The present invention relates generally to balance chamber type sealing apparatus and, more particularly, but not by way of limitation, to sealing apparatus suitable for use between a downhole motor and pump.
2. Brief Description of the Prior Art
In the recovery of oil from a well, it is often necessary that fluids in the well be pumped to the surface and various types of pumping units have been developed for this purpose. A particular type of unit which is advantageous in some circumstances comprises an electric motor and a pump which can be operated by rotating an input shaft thereof. In these units, it is common practice to connect the output shaft of the motor to the input shaft of the pump and lower the entire assembly into the well.
While pumping units of this type offer certain advantages known to those skilled in the art, they also present technical problems. A basic problem is to isolate the interior of the motor from well fluids while, at the same time, providing for repeated expansion and contraction of oil with which the motor is filled, such expansion and contraction arising thermally from intermittent operation of the motor.
A solution to this problem is to incorporate a balance chamber into a seal section disposed between the motor and the pump. The upper end of the chamber is placed in fluid communication with the interior of the motor while the lower end thereof is placed in fluid communication with the exterior of the seal section; that is, at such times that the pumping unit is in the well, with fluids to be pumped from the well. The chamber is filled with the same oil as that which fills the motor or, in some cases, with a combination of fluids prior to lowering the pumping unit into a well. When the motor is initially operated, the oil in the motor expands with the result that a portion of the fluid in the balance chamber is discharged into the well. Upon subsequent cooling and contraction of the oil in the motor, when the motor is shut off, well fluids enter the balance chamber at the lower end thereof. However, such fluids are prevented from entering the motor by selecting the fluids with which the motor and chamber are filled to be immiscible with the well fluids and to be less dense than the well fluids. With these selections, well fluids that enter the balance chamber will underlie the fluids that are introduced thereinto and into the motor so that, if the balance chamber has sufficient volume, the well fluids will not reach the upper end of the balance chamber when the motor is turned off, causing oil therein to contract, so that no well fluid will reach the motor. Moreover, after the initial operation of the motor, subsequent operations will cause well fluids which have entered the balance chamber to be discharged therefrom prior to further discharge of fluids which were initially introduced into the motor and chamber because of the discharge of fluid from the lower end of the balance chamber. As a result, subsequent operations of the motor merely move the interface between the well fluids and the fluids introduced into the motor and balance chamber between two levels in the balance chamber. Examples of seal sections designed to be used to connect an electric motor to a pump in a downhole pumping unit and incorporating a balance chamber to provide a seal as described above are disclosed in U.S. Pat. No. 2,783,400 issued Feb. 26, 1957 to Arutunoff; in U.S. Pat. No. 3,150,160 issued Oct. 13, 1964 to Carle; and in U.S. Pat. No. 3,502,919 issued Mar. 24, 1970 to Boyd, et al.
While the concept of the balance chamber is straight forward, practical problems are encountered in their use. In particular, one problem which has heretofore remained unsolved is associated with a dependence on the requisite volume of the balance chamber on the circumstances in which a particular downhole pump will be used. As a practical matter, considerable variety exists in these circumstances and, consequently, a corresponding variety exists in the sizes of the motors that are incorporated into downhole pumping units. Since different size motors will, through thermal expansion of oil contained therein, discharge different volumes of oil upon initial startup, the requisite volume of the balance chamber for the above described operation will depend upon the size of the motor with which it is used. While it is possible to make a balance chamber large enough to accommodate a variety of motor sizes, it is uneconomical to do so and, moreover, the construction of large balance chambers can give rise to another difficulty. An increase in the size of the balance chamber generally must be effected through an increase in its length with the result that the connection between the motor and the pump can present a problem. Such connection is made via a shaft which extends through the balance chamber and increasing the length of the shaft introduces a tendency of the shaft to undergo a whipping motion as it rotates to drive the pump. This motion can decrease the lifetime of seals mounted on the shaft to seal the upper end of the balance chamber against the entry of well fluid thereinto along the shaft. While this problem can be eliminated by supporting the shaft in its passage through the balance chamber, such support often makes it difficult to assemble the seal section which contains the balance chamber.